1. Field of the Invention
The present invention relates generally to a method of calibration and more particularly to a calibration method and device for use in long range guided wave inspection of piping.
2. Description of the Prior Art
The use of long range guided wave techniques for inspection of large areas of structures, including piping, from a single sensor location is an emerging technology. For piping, the technique involves the launching of a pulse of guided waves along the length of the pipe. Any reflected signals are detected by a receiver. Reflected signals may be from (a) the natural structure of the pipe or (b) corrosion or cracks in the pipe.
The occurrence time of the reflected signal from the time of launch until received can be used to determine the axial location of the defect. The amplitude of the reflected signal will determine the severity of the defect.
The long range guided wave technique gives a one hundred percent volumetric inspection of a long length of piping from a single probe location. In a typical inspection over a hundred feet of pipe can be inspected at frequencies of approximately 100 kHz or less for pipes that are not coated or buried. This long range guided wave inspection technique is particularly useful for remote inspection of difficult to access areas by launching and receiving reflected waves from an accessible location. For example, the guided waves can travel along the pipe under installation or at high elevations that are inaccessible.
The use of long range guided wave techniques for comprehensive inspection of piping is gaining rapid acceptance as a cost effective inspection method in various industries, including gas, oil, petrochemical, and electric power. This is particularly true where piping is a primary component of the facility. One reason why the long range guided wave technique is gaining popularity is because of its minimum preparation and inspection time.
The magnetostrictive (MsS) sensor technology for guided waves has been extensively developed and patented by Southwest Research Institute. In the magnetostrictive techniques, Southwest Research Institute is one of the world leaders in using this technology for long range guided wave inspection. Just some of the patents owned by Southwest Research Institute in this area include U.S. Pat. Nos. 5,456,113; 5,457,994; 5,581,037; 5,767,766; 6,212,944; 6,294,912; and 6,429,650.
While Southwest Research Institutes owns other patents utilizing the magnetostrictive technology, it is believed these provide a good illustration of the prior magnetostrictive patents that exist.
For the benefit of those who do not understand the magnetostrictive effect, the magnetostrictive effect refers to a physical dimension change in ferromagnetic materials that occurs when a magnetic field is applied to the material. Mechanical waves are generated by introducing a pulse current into a transmitting coil adjacent to a ferromagnetic material that, in turn, changes the magnetization within the material located near the transmitting coil. The change in magnetization within the material located near the transmitting coil causes the material to change its length locally in a direction parallel to the applied field. This abrupt local dimension change, caused by the magnetostrictive effect, generates a mechanical wave (called a guided wave) that travels through the ferromagnetic material at a fixed speed.
When a mechanical wave is reflected back, it indicates a physical barrier, such as (a) end of the ferromagnetic material, (b) defect in the ferromagnetic material, or (c) some other geometric changes, such as welds. When the reflected mechanical wave (guided wave) reaches a detection coil, the mechanical wave causes a changing magnetic flux in the detection coil through the inverse magnetostrictive effect. This changing magnetic flux induces an electric voltage within the detection coil that is proportional to the magnitude of the reflected mechanical wave. The transmitting coil and the detecting coil can be (a) the same coil or (b) separate but identical coils.
Despite all the advances that have been made in magnetostrictive techniques, there still needs to be a simple and accurate way to calibrate the magnetostrictive inspection system. Calibration is necessary to quantify the reflected signals and relate the reflected signals to the size of a defect. Calibration is necessary to determine the scale of the reflected signal in relation to a percentage defect. In conventional inspection techniques, such as ultrasonic or eddy current, calibration may be achieved by using reflected signals from a reference reflector. The reference reflector may be the back wall, a side drilled hole, a flat bottom hole or a fixed diameter reference block. By using a known type of reflector, calibration of the scale can occur. Also another manner of calibration is by using a short piece of reference pipe with reference reflectors. However, the reference pipe has to match the pipe under test. That is not possible most of the time.
The size (and hence scale) of the reflected signal will vary according to the pipe itself. The thickness or diameter of the pipe, the physical condition of the pipe (new, rusty, etc.), as well as the material out of which the pipe is made affects the transmission of a magnetostrictive signal therethrough. Therefore, a reference signal that is used in one size pipe made of a particular material does not apply to a different size pipe made from another material, both of which may be ferromagnetic.
Also the coupling between the guided wave probe in the pipe is variable from one situation to the next with the variations being considerable in the field from pipe to pipe and from location to location. Variations can depend upon the diameter of the pipe, wall thickness of the pipe, and condition of the pipe under inspection. Calibration of the scale by using a reference pipe has been found to be impractical in most occasions. Therefore, the present invention is directed toward calibrating the scale of the reflected signal in relation to percentage defect by using guided wave signals in the pipe that is under inspection.
One method of calibration of the scale that has been used in the past is simultaneously detecting the transmitted signal and the reflected signal by using a second guided wave probe installed some distance away and using the transmitted signal as the reference. While this approach provides for fairly good calibration, it is not always practical because of the increased inspection time and distance at which the second probe must be located.
As a compromise solution, signals from girth welds in pipe have generally been used as a calibration reference. This approach, though convenient, is not reliable. The welds are not identical and their signals vary widely from pipe to pipe and from location to location. Even the skills of the welder in creating the girth weld can greatly affect the reflected signal.
All of these problems led to the need for a direct method to calibrate the scale that is simple, reliable and inexpensive for long range guided wave inspection of pipes.
This invention is needed to enhance the reliability of the results received from long range guided wave inspection of piping.